Subject: Re: the prom: to map or not to map
To: Jason Brazile <email@example.com>
From: Chris G. Demetriou <firstname.lastname@example.org>
Date: 06/11/1997 09:29:20
> > If the alpha happens to need prom services (printf, in particular), it
> > temporarily maps it in.
> Hmm, so I guess you always have to leave "room" or whatever resources
> are necesary to be able to map it in or else the panic'ing due to
> lack of free pages would be hard to diagnose :-)
The kernel doesn't "map it in" by adding it to the kernel's mappings
(which may be more correct, actually, but is harder).
At system startup time, the firmware is living in a known location in
VA space: that covered by the first L1 PTE. Therefore, I squirrel
that away, and when necessary swap it with the 'normal' first L1 PTE
of the running process, do the firmware access, then swap it back.